1. Field of the Invention
The present invention relates to a reference-voltage circuit using a band-gap voltage, and more particularly to temperature compensation thereof.
2. Description of the Related Art
FIG. 2A is a view of a conventional band-gap circuit, and FIG. 2B shows temperature characteristics of an output voltage of the conventional band-gap circuit shown in FIG. 2B. The band-gap circuit shown in FIG. 2A consists of PNP-type bipolar transistors (hereinafter referred to as ‘PNP’) 1 and 2; resistors 3, 4 and 5; and a differential operational amplifier 6. A base and a collector of the PNP 1 are connected to ground, and an emitter is connected to an inverting input terminal of the differential operational amplifier 6. In addition, a base and a collector of the PNP 2 are connected to ground, and an emitter is connected to a non-inverting input terminal of the differential operational amplifier 6 through the resistor 5. An output terminal of the differential operational amplifier 6 is connected to the inverting input terminal through the resistor 3 and the non-inverting input terminal through the resistor 4 as well. Furthermore, a constant voltage VBG is output from the output terminal of the differential operational amplifier 6.
If the base-emitter voltage of the PNP 1 shown in FIG. 2A is defined as VBE, resistances of the resistors 3, 4 and 5 are defined as R3, R4 (=m×R3) and R5 respectively, and the area ratio between PNP 1 and the PNP 2 is 1:n, then the output voltage VBG may be represented by the following equation:VBG=VBE+mXR3/R5×VTX  (1).In equation (1), VT is thermoelectric voltage (=kT/q, wherein k is Boltzmann constant, T is absolute temperature, and q is electric charge) and has a positive temperature coefficient of around 0.0086 mV/° C. Also, X=m×n. Meanwhile, VBE as the first term of equation (1) has a negative temperature coefficient of around −2 mV/° C. Therefore, an output voltage independent from the temperature can be obtained by setting m, n, R3 and R5 so that the first and second terms of equation (1) cancel each other.
However, the base-emitter voltage VBE of a transistor used in practical circuits includes a nonlinear component in its temperature characteristics, so that the temperature coefficient is not constant. Therefore, the output voltage VBG of a practical band gap circuit has curved temperature characteristics including a peak value or a bottom peak, as shown in FIG. 2B. In addition, the issue of whether the temperature has a peak value or a bottom value depends on the production process of transistors and resistors composing the circuit.